The oil pump
is the heart of the lubricating system; it forces oil out of the oil pan, through the oil
filter, galleries, and to the engine bearings. Normally, a gear on the engine camshaft
drives the oil pump; however, a cogged belt or a direct connection with the end of the
camshaft or crankshaft drives the pump in some cases.
There are two
basic types of oil pumpsrotary and gear.
pump (fig. 6-21)
has an inner rotor with lobes that match similar shaped depressions in the outer rotor.
The inner rotor is off center from the outer rotor.
As the oil
pump shaft turns, the inner rotor causes the outer rotor to spin. The eccentric action of
the two rotors forms pockets that change size. A large pocket is formed on the inlet side
of the pump. As the rotors turn, the oil-filled pocket becomes smaller, as it nears the
outlet of the pump. This action squeezes the oil and makes it spurt out under pressure. As
the pump spins, this action is repeated over and over to produce a relatively smooth flow
The GEAR pump
consists of two pump gears mounted within a close-fitting housing. A shaft, usually turned
by the distributor, crankshaft, or accessory shaft, rotates one of the pump gears. The
gear turns the other pump gear that is supported on a short shaft inside the pump housing.
As a safety
factor to assure sufficient oil delivery under extreme operating conditions, the oil pump
(gear or rotary) is designed to supply a greater amount of oil than is normally required
for adequate lubrication. This requires that an oil pressure relief valve be incorporated
in the pump to limit maximum oil pressure.
relief valve is a spring-loaded bypass valve in the oil pump, engine block, or oil filter
housing. The valve consists of a small piston, spring, and cylinder. Under normal pressure
conditions, the spring holds the relief valve closed. All the oil from the oil pump flows
into the oil galleries and to the bearings.
under abnormally high oil pressure conditions (cold, thick oil, for example), the pressure
relief valve opens. Oil pressure pushes the small piston back in its cylinder by
overcoming spring tension. This allows some oil to bypass the main oil galleries and pour
back into the oil pan. Most of the oil still flows to the bearings and a preset pressure
is maintained. Some pressure relief valves are adjustable. By turning a bolt or screw or
by changing spring shim thickness, the Oil on the inlet side of the pump is caught in the
gear teeth and carried around the outer wall inside the pump housing. When oil reaches the
outlet side of the pump, the gear teeth mesh and seal. Oil caught in each gear tooth is
forced into the pocket at the pump outlet and pressure is formed. Oil squirts out of the
pump and to the engine bearings. pressure setting can be altered.
Figure 6-21.Rotor-type oil pump.
Figure 6-22.Gear-type oil pump.